Electrical cables for data transmission are well known. One common cable is a coaxial cable. Coaxial cables generally comprise an electrically conductive wire surrounded by an insulator. The wire and insulator are surrounded by a shield, and the wire, insulator and shield are surrounded by a jacket. Coaxial cables are widely used and best known for cable television signal transmission and ethernet standard communications in local area networks. Coaxial cables can transmit at much higher frequencies than a standard twisted pair wire and, therefore, have a much greater transmission capacity. In addition, coaxial cables have very little crosstalk, and therefore, provide a very reliable medium for data transmission. Other types of cables are also well known, such as twisted pair cables used for telephone signal transmission, and fiber optic cables.
With the proliferation of high-speed, powerful personal computers and the availability of advanced telecommunications equipment, there is a need for cables which are capable of transmitting data at ever faster speeds. Fiber optic cables provide optimum data rate and performance for long distance and high data rate transmissions, since fiber optic cables provide very high data rate transmission with low attenuation and virtually no noise. Fiber optic cables provide data transmission at data rates up to and beyond 10 Gbps. However, despite the increased availability of fiber optic cables, the price of fiber optic cables and transceivers have not dropped to a level where it is always practicable to use. Accordingly, other less expensive cables capable of high speed data transmission are still in demand.
One such cable used for high speed data transmission between two points or devices is a shielded parallel pair cable. Parallel pair cable designs provide two separately insulated conductors arranged side by side in parallel relation, the pair being then helically wrapped in a shield. However, a helically applied shield has many discontinuities. The signal path within the shield is interrupted each time the signal encounters an overlap in the spiral. These repeated interruptions cause signal loss, measured as increased attenuation. A common usage of these cables is to interconnect a mainframe computer to a memory device. As is well known, the speed and data rate with which the computer must communicate with the memory is critical to the computer's performance capabilities. Parallel pair cables are usually used for differential signal transmission. In differential signal transmission, two conductors are used for each data signal transmitted and the information conveyed is represented as the difference in voltage between the two conductors.
U.S. Pat. No. 6,677,518 to Hirakawa et al. discloses a data transmission cable having a pair of conductors, each coated with an insulation, and a drain wire. The pair of conductors and drain wire are successively covered by a shielding tape, which can be longitudinally or spirally wrapped, having a thickness of between 1 μm and 10 μm; and a resin layer. There is no disclosure of polymeric tape layers intermediate to the shielding tape and resin layer.
U.S. Pat. No. 5,483,020 to Hardie et al. discloses parallel pair cable having a pair of conductors, each covered with an insulation to electrically insulate the conductors from each other. The insulated conductors are then covered with a metal shield, preferably constructed of a plurality of interwoven electrically conductive strands to prevent radiated energy from escaping the cable construction. The shield is surrounded by a jacket to protect the cable. There is no disclosure of polymeric tape layers intermediate to the shielding tape and resin layer or of a drain wire.
Even with recent advances in parallel cable construction, there remains a need for a high speed parallel pair cable that reduces skew and also minimizes the pair shield discontinuities in order to achieve low insertion loss performance.